When a voice signal is transmitted in a packet communication system typified by Internet communication, a mobile communication system, or the like, compression and coding technology is used to increase transmission efficiency. To date, many voice coding methods have been developed, and many of the low bit rate voice coding methods developed in recent years have a scheme in which a voice signal is separated into spectrum information and detailed spectrum structure information, and compression and decoding is performed on the separated items.
Also, with the ongoing development of voice telephony environments on the Internet as typified by IP telephony, there is a growing need for technologies that efficiently compress and transfer voice signals.
In particular, various schemes relating to voice coding using human auditory masking characteristics are being studied. Auditory masking is the phenomenon whereby, when there is a strong signal component contained in a particular frequency, an adjacent frequency component cannot be heard, and this characteristic is used to improve quality.
An example of a technology related to this is the method described in Non-Patent Literature 1 that uses auditory masking characteristics in vector quantization distance calculation
The voice coding method using auditory masking characteristics in Patent Literature 1 is a calculation method whereby, when a frequency component of an input signal and a code vector shown by a codebook are both in an auditory masking area, the distance in vector quantization is taken to be 0.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI 8-123490 (p. 3, FIG. 1)